Tracy Reiner In 'Shock' Over Deaths of Her Father Rob and His Wife

Hollywood and fans are reeling over the killing of Rob Reiner and his wife Michele ... and the family is speechless, finding it difficult to comprehend the news.

TMZ broke the story ... the legendary actor-director and his wife were slain in their Brentwood home Sunday ... and we've learned the couple's son Nick was in law enforcement custody as of Monday morning, being questioned about their deaths.

Tracy Reiner -- Rob's adopted daughter from his marriage to ex-wife Penny Marshall -- told NBC News ... “I came from the greatest family ever. I don’t know what to say. I’m in shock.”

The deceased couple was discovered inside the L.A.-area home by one of their daughters ... and she told police a family member had killed them.

Our sources also say the daughter told police the family member "should be a suspect" because they're "dangerous."

TMZ broke the story ... Rob and Michele suffered lacerations consistent with knife wounds, and LAPD's Robbery Homicide Division is investigating the case.